If you have a supported Google Pixel device, then you'll get Android 12 on day 1 of the release. So, if you run one of the following devices, you're in the club.
Pixel 4a 5G
Pixel 4 XL
Pixel 3a XL
Pixel 3 XL
Beyond that, things get hazy.
The beta runs on a limited number of devices, but if and when the final release rolls out to non-Google devices is up to the OEM that made the device. It could be soon after the release, or it could be many months down the line.
Or it could be never.
Then there's something called performance class. Starting with Android 12, Google is kicking off this standard as a way to define "a set of device capabilities that goes beyond Android's baseline requirements." Along with "performance class 12," which will define devices that can run Android 12, Google will also publish a "performance class 11" that will be a subset of the higher class.
Devices that conform to "performance class 11" will be able to run Android 12, but won't be able to get access to all the features.
So, if you have a supported Google Pixel device, you're in. If not, you're at the whim of both the new "performance class" rating and the OEMs. Some OEMs are quite quick to roll Android 12 to supported devices (such as Asus), while OEMs that do more substantial tweaks (think Samsung and Xiaomi) take longer.